Though Senate passage of a $410 billion omnibus spending bill seemed all but assured at the end of last week, Democratic defections and only modest GOP support threaten to make this week’s expected vote on final passage a squeaker.

“I think it’s fair to say that there’s an extraordinary amount of pressure on this particular bill,— Sen. Claire McCaskill (Mo.) said.

McCaskill is one of a handful of Democrats who has said she’s undecided about backing the bill, given that it increases spending by 6 percent over last year and contains thousands of earmarks, or targeted funding for lawmakers’ pet projects. Two Democrats — Sens. Russ Feingold (Wis.) and Evan Bayh (Ind.) — have said they will vote against the measure and encouraged President Barack Obama to veto it.

Meanwhile, Republican support is thin, with Democrats struggling to find as many as six GOP Senators to vote for the bill and reach a filibuster-proof, 60-vote threshold. Democratic aides estimated they already have three or four GOP Senators who are likely to vote with them.

“We’re working on it. I hope at the end of the day we don’t lose— as many as four of five Democrats, Majority Whip Dick Durbin (D-Ill.) said. “At this point, a number of Republicans have expressed interest in voting for it, but not [given] a commitment, so we’re trying to work through that.—

McCaskill, who voted this week with Republicans on amendments to trim the bill’s price tag and eliminate earmarks, said Democratic leaders are working overtime to lobby the Conference to support the bill.

“I’m going to continue to talk to people and be open to people. And you know, I am more comfortable now that I’ve had the opportunity to express my displeasure through these amendments,— she said.

Besides McCaskill, Democrats on the fence include Sens. Bob Menendez (N.J.), a Cuban American who opposes provisions that would ease travel restrictions to Cuba, Ben Nelson (Neb.) and Bill Nelson (Fla.).

That problem might usually be solved by adopting amendments to satisfy reluctant Members. But the Democratic leadership’s refusal to accept changes to the bill has left both Democrats and Republicans fuming and is responsible, at least in part, for the difficulty that they are having in getting 60 votes to overcome GOP objections to ending debate on it. Reid filed a motion Wednesday to limit debate, which sets up the key test vote for Friday.

“We’re not accepting Democratic amendments either — at least that’s not our goal — and a number of people want to offer them,— Durbin said. “But if we do that, it’s over, we’re going to be on a continuing resolution until September.—

Majority Leader Harry Reid (D-Nev.) has said no changes can be made because the stop-gap spending bill, or continuing resolution, which has been funding the government since this fall, expires Friday. Failure to pass another funding bill of some sort would result in a government shutdown. Reid has said there is not enough time between now and Friday to convene a House-Senate conference committee on the bill if the Senate makes any changes.

Reid added that when he spoke to Speaker Nancy Pelosi (D-Calif.) on Wednesday, she flatly refused to allow the House to approve any Senate changes to the measure that her chamber passed last week.

“Just so there’s no misunderstanding … [Pelosi] said that, We put our Members through a lot over here on this appropriations bill. I’m not going to put them through anymore. If there are any amendments to this, we’re going to do a CR for the rest of the year,’— Reid said on the Senate floor.

But Sen. Tom Coburn (R-Okla.) said Democratic leaders’ arguments that changes to the measure will doom the bill don’t hold water. He has pushed strongly to strip out earmarks in the bill as well as ensure that funds are disbursed under competitive bidding requirements. Still, Coburn lost votes on all four of his amendments Wednesday.

“This amendment won’t kill this bill,— Coburn said on the floor of one of his proposals. “What it’ll do is go back to the House, and they’ll have to agree to it. Everybody knows that.—

The threat of another CR, which would prevent Congress from setting new funding priorities for federal agencies, may end up swaying some Republicans who are on the fence.

In a crucial vote Tuesday, nine Republicans — most of them on the Appropriations Committee — voted against a GOP leadership-blessed amendment to simply pass a CR through the end of the fiscal year.

“I voted against the CR because a CR is a bad way to run a government,— said GOP Conference Chairman Lamar Alexander (Tenn.), who has not yet made up his mind about whether to vote for final passage of the omnibus.

Similarly, Sens. Susan Collins (R-Maine) and Lisa Murkowski (R-Alaska) — both of whom voted against the CR amendment —said they also are undecided on the final vote.

However, Sen. Bob Bennett (R-Utah), a conservative who sided with Democrats against the CR, said he could not vote for the bill as it currently stands because the $410 billion total is “too high.—

Democratic aides said the Republicans who are likely to vote for the omnibus are Senate Appropriations ranking member Thad Cochran (Miss.) and Sens. Kit Bond (Mo.) and Richard Shelby (Ala.). Bond and Shelby also sit on the Appropriations Committee. Plus, moderate Sen. Olympia Snowe (R-Maine), who is not an appropriator, told reporters Wednesday that she is likely to support it.

Even Republican Policy Committee Chairman John Ensign (Nev.) said he didn’t envision a scenario where Republicans would be able to kill the omnibus.

“I don’t see the votes unfortunately,— Ensign said.

John Stanton contributed to this report.